Achieving room-temperature continuous-wave (RTCW) vertical cavity laser (VCL) operation at wavelengths beyond about 3.0 microns (um) presents severe challenges. As of June 2017, electrically pumped VCLs (eVCLs) employing type II interband cascade laser (ICL) technology, though promising, had achieved only room temperature pulsed operation above 3.0 um. Two state of the art results are described in “Room-temperature Mid-Infrared Interband Cascade Vertical Cavity Surface Emitting Lasers,” by W. W. Bewley et al in Applied Physics Letters 109, 151108 (2016), and in “Room-temperature vertical cavity surface emitting lasers at 4 um with GaSb-based type II quantum wells,” by G. K. Veerabathran, et al in Applied Physics Letters 110, 071104 (2017). Achieving RTCW operation in ICL eVCLs will require further reduction of operating voltages, and/or reduced optical losses.
If alternative type I InGaAsSb quantum wells for eVCLs are employed, a different set of challenges emerges. The band line-up of type I quantum wells with either AlGaAsSb or AlInGaAsSb barriers lattice-matched to GaSb leads to increasingly poor hole confinement with increasing wavelength, resulting in reduced material gain and reduced maximum operating temperature. This challenge is described in “Type I Diode Lasers for Spectral Region Above 3.0 um,” by G. Belenky, et al, IEEE Journal of Selected Topics in Quantum Electronics, vol. 17. No. 5, September/October 2011. This problem is even more severe in VCLs than in edge-emitting lasers, since VCLs have short gain length and generally worse thermal impedance than edge-emitters. Thus, RTCW operation has also not yet been achieved beyond 3.0 um in VCLs employing type I quantum wells.
From the foregoing, it is clear that what is required is a vertical cavity laser structure operating at a wavelength >3.0 um, which is capable of room temperature continuous wave operation.